Direct current (DC) output active power factor correction circuits are known in the art. However, prior to the present invention, alternating current (AC) output active power factor correction circuits were not considered viable. Accordingly, this invention relates to modifying the prior art DC circuits to provide an AC output.
In DC output circuits of the type described, a bulk energy storage capacitor is used and charging current into the bulk energy storage capacitor is proportional to the square of the instantaneous AC input line voltage. The current extracted from the AC line is proportional to the instantaneous AC line voltage.
In operation, the AC input or line voltage is full wave rectified by a diode bridge rectifier to provide a rectified sine wave or unfiltered DC output. This output is applied to the input of a switching regulator which is typically a boost converter having an output which is greater than the highest peak input voltage. The feedback loop which regulates the output voltage is modified by adding circuitry to multiply the output of its error amplifier by a sample of the rectified AC input voltage, and comparing this with the sensed current. The resultant signal is used to control the instantaneous duty-cycle of the switching regulator. This causes the input current waveform to follow the input voltage waveform, resulting in low harmonic distortion and a current waveform which is in phase with the voltage waveform.
In order to provide an AC output, the arrangement described above is modified in accordance with the present invention as will be hereinafter described and features a bi-directional bridge rectifier followed by a bi-directional switching regulator. This is in contrast to the arrangement disclosed and claimed in the aforenoted U.S. application Ser. No. 08/000,864 (File No. 550-92-008) which features two bi-directional switching regulators for providing the AC output, and to the arrangement disclosed and claimed in the aforenoted U.S. application Ser. No. 08/000,863 (File No. 550-92-009) which features a capacitor coupled bi-directional switching regulator for providing said AC output.